1. Field of the Invention
The present invention relates to a V-groove shape measuring method and apparatus, and more particularly to a V-groove shape measuring method and apparatus preferably used for measuring characteristic values such as pitch deviation or axial runout of side face of V-grooves of the work to be measured spirally forming V-grooves for worm gear, male threads, screw holes, and the like, by using a three-dimensional coordinate measuring machine.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
As an instrument for measuring a solid figure of a work or other object to be measured at high precision, a three-dimensional coordinate measuring machine (or three-dimensional measuring machine) 10 as shown in FIG. 1 has been known. It comprises a platen 12 with a smooth and flat polished surface made of, for example, granite, a portal frame 14 installed to be movable in the longitudinal direction (for example, Y-axis direction) of the platen 12, a slider 16 installed to be movable in the lateral direction (for example, X-axis. direction) along a horizontal beam 15 of the portal frame 14, an elevating shaft 18 installed on this slider 16 to be movable in the vertical direction (for example, z-axis direction), and a probe 22 for measuring coordinates, being provided at the lower end of this elevating shaft 18 through a probe holder 20, and having a measuring element 24 formed at the leading end thereof.
Therefore, after putting a work W to be measured on the platen 12, the probe 22 is moved in three directions (X-, Y-, Z-axis directions) to cause the measuring element 24 to contact with measuring positions of the work W, and at each contact point, sequentially, the coordinate values in each axial direction of the probe 22 are read from a scale not shown, and these coordinate values are calculated, so that the dimensions and angle of the work W can be measured at high precision.
As a method of measuring 1 the screw shape by using such three-dimensional measuring machine, the present applicant proposed a method of determining the center coordinates of screw hole in, for example, Japanese Laid-open Patent No.Hei 6(1994)-341826.
When measuring the work shape by using such three-dimensional measuring machine, it is required to scan and measure the work coordinates sequentially while moving the probe 22 for measuring the coordinates. As an automatic method for this scanning control by using a computer, without using a rotary table, a control method of scanning along the contour of the work while keeping constant the height from a reference plane by using an arbitrary plane as a reference plane (hereinafter called height constant scanning control), and a control method, shown in FIG. 3 in case of using a rotary table 30, of scanning along the contour of the work W within a cylindrical plane determined by specification of an arbitrary straight line and the distance (that is, the radius) from the straight line (hereinafter called radius constant scanning control).
Further, as shown in FIG. 2, with using the rotary table 30, the applicant proposed U.S. Pat. No. 5,204,824 (corresponding to UK 2237661, DE4027339A1), in which one axis by rotary table is added to a three-axis scanning control without using a rotary table, scanning by a four-axis simultaneous control is realized while keeping constant the direction of the probe with respect to the measurement reference line of the work (called measuring element direction constant scanning control).
According to this method, when not using the rotary table, as in the case of the cylindrical cam shown in FIG. 3, if it is impossible to measure by one operation due to interference of the work and the probe, it is possible to measure the whole circumference by one operation without changing the position (direction) of the probe. Besides, even in the case of impeller or propeller blades that cannot be measured by one operation, the number of times of changing the position of the probe can be decreased.
Recently, on the other hand, as the objects of measurement are diversified, there are increasing demands for measuring a deviation (for example, maximum deviation) of thread pitch P of worm gear forming spiral V-grooves, general male threads, or screw holes formed in works as shown in FIG. 5, or the deviation (for example, maximum deviation xcex94R) in the radius R direction of the plane locus of threads superposed in the axial direction as shown in FIG. 6, but they could not be measured accurately in the conventional methods.
The invention is devised to solve the problems of the prior arts, and it is hence an object thereof to measure the characteristic values of V-groove shape accurately, such as worm gears, general male threads, and screw holes.
The invention is for scanning and measuring a V-groove by using a scanning probe for measuring the position, while rotating a work by a rotary table, with the work having a spiral V-groove fixed on the rotary table, by combining:
a measuring element direction constant control aiming at keeping constant a vector projected on the table plane of the rotary table of a direction vector from the origin of the work to a measuring element of the scanning probe as seen from a machine coordinate system,
a rotary table radius constant scanning control of which confinement plane is a cylindrical plane, and
a two-flank contact scanning control for causing the measuring element to contact with two flanks for composing the V-groove,
wherein a V-groove rotary table scanning control and a measurement are performed while keeping the measuring element always in contact with the two flanks for composing the V-groove, thereby solving the problems.
The V-groove rotary table scanning control may be realized by:
sampling a position vector X of the scanning probe (hereinafter all vector symbols are omitted to avoid complication of description of the specification), its displacement amount xcex94X, and rotational angle xcex8 of the rotary table,
calculating an approach reverse direction vector Qu in a direction vertical to the axial center V of the object from the rotational angle xcex8 of the rotary table,
calculating the speed vector V of the probe while the rotary table is stopped at the rotational angle xcex8,
calculating the angular velocity xcfx89W of the rotary table by speed vector V of the probe as seen from the axial center of the work,
adjusting the advance or retardation from a target value of the rotational angle xcex8 of the table due to control error from the configuration of the table rotational angle xcex8 and the probe position X, determining a correction angular velocity xcex94xcfx89, and correcting the angular velocity cow from this xcex94xcfx89,
calculating a speed vector Vt following up the movement of the correction angular velocity xcex94xcfx89 at the probe position X and the table rotational angle xcex8, and
calculating a vector sum Vf(=V+Vt) of the follow-up speed vector Vt and probe speed vector V to obtain a probe speed command, and correcting the angular velocity xcfx89w by the correction angular velocity xcex94xcfx89 to obtain a value xcfx89t(=xcfx89w+xcex94xcfx89) as a speed command of the rotary table.
The speed vector V of the probe may be the sum of the basic speed vector Vo showing the basic running direction of the scanning probe, a displacement correction vector Ve for keeping constant the displacement of the scanning probe, and a two-flank contact vector Vh for causing the measuring element to contact with two flanks of the V-groove.
Further, the radius correction vector Vr for keeping the radius constant may be added to the sum to obtain the speed vector V of the probe.
It may be regarded as an error when two-flank contact is not maintained during two-flank contact scanning control, so that a wrong measurement may not persist.
It may be judged that the two-flank contact is not maintained when an angle xcex1 formed by a vector Es projecting a probe normal vector Eu on the plane formed by the approach reverse direction vector Qu in the reverse direction of the approach direction to the work of the scanning probe and the vector gxcex8 corresponding to the axial center of the work, and the approach reverse direction vector Qu becomes larger than a specified value.
It may be designed to obtain a two-flank contact securely before start of measurement by conducting approach process for causing the measuring element of the scanning probe to contact with the two flanks for composing the V-groove of the work before starting V-groove rotary table scanning control.
The approach process may be conducted by
moving the work and the scanning probe relatively by the relative speed vector V obtained by the sum of a displacement correction vector Ve for keeping constant the displacement of the scanning probe, and a two-flank contact vector Vh for causing the measuring element to contact with the two flanks of the V-groove, and
stopping the probe by judging that the two flanks are brought into contact when the angle xcex1 formed by the vector Es projecting the probe normal vector Eu on the plane formed by an approach reverse direction vector Qu in the reverse direction of the approach direction to the work of the scanning probe and the vector gxcex8 corresponding to the axial center of the work, and the approach reverse direction vector Qu becomes within a specified value.
Further, processing in the approach direction of the scanning probe in the approach process and processing in the approach direction of the scanning probe in the approach process in the V-groove rotary table scanning control may be common, thereby starting the rotational angle xcex8 of the rotary table from other than reference xcex8=0xc2x0 can be permitted.
Further, when approaching from a direction of a certain axis of the machine coordinate system, other axes may be clamped, and effects of friction may be eliminated.
Further, the measurement may be conducted when the central axis of the work and the central axis of the rotary table are not matched with the specified range.
It may be judged that the central axis of the work and the central axis of the rotary table are matched when the distance from the origin of the work coordinate system having the central axis of the work as the third axis to the central axis of the rotary table, and the angle formed by the vector obtained by projecting the third axis of the work coordinate system on the plane including both origin of the work coordinate system and the central axis of the rotary table and the central axis of the rotary table, both settle within the specified allowable range respectively, and judged that the central axis of the work and the central axis of the rotary table are not matched otherwise.
In the invention, fixing the work forming a spiral V-groove on the rotary table, if the central axis of the work and the central axis of the rotary table are matched within the specified allowable range, in the case of scanning measurement of the V-groove by using the scanning probe for measuring the position while rotating the rotary table, by combining:
two-flank scanning control for causing the measuring element of the scanning probe to contact with the two flanks for composing the V-groove, and the pitch scanning control for moving the measuring element in the central axis direction at a speed determined on the basis of the pitch of the V-groove and the rotating speed of the rotary table,
the V-groove rotary table scanning control for measuring while keeping the measuring element always in contact with the two flanks for composing the V-groove is executed, thereby solving the problems.
The V-groove rotary table scanning control is executed by:
sampling the position vector V of the scanning probe, its displacement xcex94X, and rotational angle xcex8 of the rotary table,
calculating xcfx89 so that the composite speed of a peripheral speed Vxcfx89(=rxcfx89) produced on the basis of the distance r from the central axis of the rotary table to the position vector X, and a speed vector Vz(=GP(2xcfx80/xcfx89)) in the central axis direction of the rotary table produced on the basis of a specified screw pitch GP when the rotary table is rotated at the angular velocity xcfx89 may be the specified scanning speed V,
calculating a speed vector Vz on the basis of this xcfx89 value and screw pitch GP, and
a setting the speed vector Vz as a speed vector command Vt to the scanning probe, and this xcfx89 value as the rotational speed command of the rotary table.
The speed vector command Vt of the probe may be the sum of the speed vector Vz, and a displacement correction vector Ve for keeping constant the displacement of the scanning probe.
The invention solves the problem by presenting the V-groove shape measuring apparatus which comprises a rotary table fixing a work forming a spiral V-groove, a scanning probe having a measuring element engaged with the surface of the work, a drive mechanism for moving the scanning probe along the surface of the work, position detecting means for detecting the position of the scanning probe, and control means for controlling the moving speed of the scanning probe and rotating speed of the rotary table so that the measuring element may always contact with the two flanks for composing the V-groove, by any one of the methods described above.
Plural measuring elements differing in diameter may be disposed parallel to the scanning probe, so as to be selected according to the size of the V-groove, and therefore it is easy to cope with changes of screw shape.
The rotary table may be assembled into a three-dimensional measuring machine, and its coordinate measuring probe is used as the scanning probe, so that the shape of the V-groove can be measured by the three-dimensional measuring machine.
By drilling a hole in the center of rotation of the rotary table or in the platen of the three-dimensional measuring machine immediately beneath it, the lower end of a long work is received, so that the measuring range of the three-dimensional measuring machine may not be limited by the rotary table or the parts not required to be measured beneath the work.
According to the invention, the pitch, axial runout and so on of side face of the V-groove of the work formed in a spiral profile can be measured accurately.